Propulsion systems for machines typically comprise one or more power units, such as gas turbines and/or reciprocating engines. Such machines include vehicles, such as marine vessels, aircraft or land vehicles. The power output from the power units may, for example, provide the required loads to house loads, operator quarters or work tools and, if work machine is a vehicle, provide a thrust for enabling the machine to move. The range of potential power output from the power unit may be selected in order to meet the expected loads required in the operating profiles of the work machine. There are also environmental and cost reduction benefits in operating such power units efficiently. In particular, the power unit may be selected to operate at maximum efficiency at the vehicle cruising speed, which is the speed at which the vehicle moves for the majority of the time it is operational. Marine vessels typically comprise either a mechanical or an electrical propulsion system. In a mechanical propulsion system power may be transferred between a power unit and a propeller via a mechanical transmission. An electrical propulsion system may comprise a one or more power unit(s) driving an electric generator, which feeds power to at least one electric motor to drive one or more propeller(s).
A hybrid propulsion system may be utilised to improve flexibility, redundancy and efficiency. WO-A-2014/184517 discloses a hybrid propulsion system in which at least one power unit is operably connected to a transmission via a mechanical arrangement and/or via an alternate power transfer arrangement, such as a hydraulic or electric arrangement.
The transmission may be a planetary gearbox, which may enable the transmission to combine power from the mechanical and alternate power transfer arrangements or to receive power from the mechanical arrangement and supply power to the alternate power transfer arrangement. The transmission may provide a power output for propelling a vehicle.